The need for supplemental amplification for telephone receivers has been recognized by the introduction of products into the marketplace such as feature telephones and public pay telephones with built-in adjustable receive amplification, as well as replacement handsets and in-line modules which contain manually adjustable receive amplifiers. These products are beneficial for people with normal hearing in noisy environments, as well as for people with impaired hearing in normal environments. However, there are a number of shortcomings in providing receive amplification that the prior art has not satisfactorily resolved in the over twenty five years since amplified handsets have been available. Built-in receive amplifiers typically provide up to 12 dB of gain. Most replacement handsets and in-line modules provide gain up to 20 dB. An additional five to ten decibels of gain is desirable for hearing reasons, but not for other unpleasant side effects of high gain. Receive gains around 13 dB can cause a condition known as "sing" to occur when a handset is placed on a hard, acoustically reflecting surface, such as a glass table-top. Sing is a self-sustained, audible oscillation that occurs when the acoustic-to-acoustic loop gain, in part provided by the supplemental receive amplifier, is greater than unity. Sing manifests itself in shrill audible tones which are usually uncomfortable to both the near-end talker and far-end listener.
Increased receive gain normally results in increased sidetone. Sidetone is the part of the near-end talker's voice that couples into the near-end receive circuit through the telephone hybrid network as a result of mismatched network and loop impedances. A certain amount of sidetone is desirable to give the effect of a "live" telephone circuit; however, when a weak receive signal is amplified to a satisfactory level, sidetone is usually amplified to an uncomfortable level. It is well known that talkers tend to subconsciously speak more softly when sidetone is increased. Even so, spontaneous, loud expressions by the near-end talker, such as laughter, can be disconcerting. For this reason, users usually set receive gain at the minimum level needed to compensate for incoming receive levels that vary because of call-to-call differences in transmission conditions and far-end talking levels.
When one talks in a normal level after unwittingly picking up a handset having receive gain set at a high level by a prior user, one can be beset with a sudden, uncomfortably loud sidetone/receive signal. A solution to this problem is addressed in U.S. Pat. No. 4,466,120 issued to Walker, Jr. et al. which automatically resets the receiver gain to nominal when the handset is restored to its switchhook cradle. The Walker solution is burdened by the inconvenience of having to re-establish a new listening level upon each use of the telephone.
For the above stated reasons, manual adjustment of the receive gain is frequently needed.
The well-known, half-duplex technology of voice-switching complementary gains in transmit and receive channels has been used over the years in free-air speakerphone applications to avoid sing. However, in an application where the receiver is close to the ear, as is the case with a handset, voice switching sounds unnatural and distracting because the handset user hears "pumping" of both sidetone and incoming receive levels when the receive channel makes gain transitions. Full-duplex technology, such as with adaptive echo-cancellation, can mitigate these problems, but has the disadvantage of complexity and high costs.
U.S. Pat. No. 4,536,888, issued to Donald R. Wilson, Aug. 20, 1985, teaches receive signal "conditioning" that superficially resembles the instant invention. However, the Wilson patent aspires to provide receive amplification while mitigating acoustic shock. It does not address solutions to sidetone and sing problems, nor does it fully satisfy the need for avoiding frequent gain control adjustments. The Wilson patent provides linear compression (attenuation) or expansion (amplification), as needed to keep output receive signals at about a constant level. This presumably would avoid the need for frequent gain adjustments. However, when the gain is set to provide high amplification, the aforementioned loud sidetone problem must necessarily be present in the invention, indeed aggravated by the automatic additional amplification of low level signals. The Wilson patent also includes voice-switched gain in the transmit channel to suppress local background noise when the talker is quiet. But, it fails to mitigate sing since the transmit and receive channels can simultaneously be at maximum gain. Emphasis of upper voice-band frequencies (2 K-3 KHz range, as herein used) is desirable for users with presbycusis, a hearing loss at the upper voice-band frequencies that affects many in the elderly population as well as younger people with a history of prolonged exposure to hearing-damaging sound levels. Current receive amplifier technology either provides no emphasis for upper voice. band frequencies or requires an inconvenient, separate "treble" control adjustment, in addition to an overall gain adjustment, such as in Radio Shack model 43-27.
A basis for the instant invention lies in the psychoacoustics of human speech and hearing along with the inherent characteristics of acoustic feedback in telephone handsets: It is known that sidetone at upper voice-band frequencies is perceived to be more irritating than at lower frequencies. The average level of the spoken male voice is highest around 400 Hz and continually decreases at higher frequencies. The typical human ear perceives tones in the 2 K-3 KHz range to be louder than at lower frequencies. (See Beranek, Acoustics, McGraw-Hill Book Company, Inc.,New York, NY, 1954, chapter 13, pp 398,408) Furthermore, receiver-tomicrophone acoustic coupling for conventional telephone handsets resting on acoustically reflecting surfaces is highest in the 2 K-3 KHz range. The implication for realizing a natural sounding, voice-switched, receive amplified handset is to minimize switching in the lower voice frequencies where most of the voice energy occurs, and to provide most of the switching in the 2 K-3 KHZ range, where sing is more of a problem and where sidetone is more irritating.
Accordingly, a broad objective of the invention is to provide a natural sounding, voice-switched receive amplifier that mitigates sidetone and sing. An additional objective is to provide a receive amplifier with a single, manual volume control that automatically emphasizes upper voice-band frequencies in response to high gain settings. A further objective is to provide a receive amplifier that suitably services a wide range of incoming receive levels at a single gain setting.